Flexible printed circuit boards are nowadays employed in a multiplicity of electronic devices such as mobile phones, radios, computers, printers and many more. They are constructed from layers of copper and a high-melting resistant thermoplastic: mostly polyimide, less often polyester. These FPCBs are frequently produced using adhesive tapes with particularly exacting requirements. On the one hand, for producing the FPCBs, the copper foils are bonded to the polyimide films; on the other hand, individual FPCBs are also bonded to one another, in which case polyimide bonds to polyimide. In addition to these applications, the FPCBs are also bonded to other substrates.
The adhesive tapes used for these bonding tasks are subject to very exacting requirements.Since very high bond performances must be attained, the adhesive tapes used are generally heat-activatable tapes, which are processed at high temperatures. These adhesive tapes must not emit volatile constituents in the course of this high temperature load during the bonding of the FPCBs, which often takes place at temperatures around 200° C. In order to achieve a high level of cohesion the adhesive tapes ought to crosslink during this temperature load. High pressures during the bonding operation make it necessary for the flowability of the adhesive tapes at high temperatures to be low. This is achieved by high viscosity in the uncrosslinked adhesive tape or by very rapid crosslinking. Moreover, the adhesive tapes must also be solder bath resistant, in other words must for a short time withstand a temperature load of 288° C.
For this reason the use of pure thermoplastics is not rational, despite the fact that they melt very readily, ensure effective wetting of the bond substrates and lead to very rapid bonding within a few seconds. At high temperatures, though, they are so soft that they tend to swell out of the bondline under pressure in the course of bonding. Accordingly there is no solder bath resistance either.
For crosslinkable adhesive tapes it is usual to use epoxy resins or phenolic resins, which react with specific hardeners to form polymeric networks. In this specific case the phenolic resins cannot be used, since in the course of crosslinking they generate elimination products, which are released and, in the course of curing or, at the latest, in the solder bath, lead to blistering.
Epoxy resins are employed primarily in structural adhesive bonding and, after curing with appropriate crosslinkers, produce very brittle adhesives, which indeed achieve high bond strengths but possess virtually no flexibility.
Increasing the flexibility is vital for use in FPCBs. On the one hand the bond is to be made using an adhesive tape which ideally is wound onto a roll; on the other hand the conductor tracks in question are flexible, and must also be bent, readily apparent from the example of the conductor tracks in a laptop, where the foldable screen is connected via FPCBs to the further circuits.
Flexibilizing these epoxy resin adhesives is possible in two ways. First, there exist epoxy resins flexibilized with elastomer chains, but the flexibilization they experience is limited, owing to the very short elastomer chains. The other possibility is to achieve flexibilization through the addition of elastomers, which are added to the adhesive. This version has the drawback that the elastomers are not crosslinked chemically, meaning that the only elastomers that can be used are those which at high temperatures still retain a high viscosity.
Because the adhesive tapes are produced generally from solution it is frequently difficult to find elastomers of a sufficiently long-chain nature not to flow at high temperatures while being still of a sufficiently short-chain nature that they can be brought into solution.
Production via a hotmelt operation is possible but very difficult in the case of crosslinking systems, since it is necessary to prevent premature crosslinking during the production operation.
The prior art further discloses, in WO 00/01782 A1, an electrically conductive, thermoplastic and heat-activatable adhesive sheet comprising                i) a thermoplastic polymer, with a fraction of from 30% to 89.9% by weight,        ii) one or more tackifying resins, with a fraction of from 5% to 50% by weight, and/or        iii) epoxy resins with hardeners, possibly accelerators as well, with a fraction of from 5% to 40% by weight,        iv) silverized glass beads or silver particles, with a fraction of from 0.1% to 40% by weight.        
A development was disclosed by DE 198 53 805 A1, with the electrically conductive, thermoplastic and heat-activatable adhesive sheet comprising                i) a thermoplastic polymer, with a fraction of at least 30% by weight,        ii) one or more tackifying resins, with a fraction of from 5% to 50% by weight, and/or        iii) epoxy resins with hardeners, possibly also accelerators, with a fraction of from 5% to 40% by weight,        iv) metallized particles, with a fraction of from 0.1% to 40% by weight,        v) non-deformable or difficult-to-deform spacer particles, with a fraction of from 1% to 10% by weight, which do not melt at the bonding temperatures of the adhesive sheet.        
In preferred embodiments the thermoplastic polymers are in each case thermoplastic polyolefins, polyesters, polyurethanes or polyamides or modified rubbers, such as nitrile rubbers in particular.
It is an object of the invention, therefore, to provide an adhesive tape which is heat-activatable, crosslinks in the heat, possesses a low fluidity at high temperatures, displays effective adhesion to polyimide and in the uncrosslinked state is soluble in organic solvents.
This object is achieved, surprisingly, by means of an adhesive tape as characterized in more detail in the main claim. The dependent claims provide advantageous developments of the subject-matter of the invention.